Many public, semi-public, and private entities are subject to threatening telephone calls of various kinds. Airlines and railroads receive bomb threats by telephone. Governmental offices and private business are subject to similar telephone calls, often relating to the purported placement of bombs in public buildings. A threatening call may be factual; more frequently, the call is a hoax. But the called party cannot determine this with any degree of certainty and is often compelled to evacuate people from a threatened area or take other protective action.
In virtually all instances, attempts to trace such threatening telephone calls are completely ineffective. A time of at least three minutes is usually required to trace a telephone call in virtually any metropolitan area. The caller, whether in a real threat or in a hoax situation, ordinarily hangs up in less than three minutes, so that tracing efforts are quite ineffectual.
Experience indicates that most threatening telephone calls, particularly hoax calls, are made from locations closely adjacent to the threatened facility and usually from public telephones. Thus, a telephone call regarding a bomb is usually made from a pay station located within a radius of one or two miles of the threatened facility. The caller desires to see the activity induced by the call, such as evacuation of an airplane or building and inspection of the threatened facility.
A similar critical communication situation occurs in connection with kidnappings and other forms of extortion. Instructions for meeting the demands of the extortionists are frequently delivered by telephone, again often from a public telephone station. Tracing such calls is usually too time-consuming to be effective.
To cope with this problem, it is highly desirable to provide for effective identification of a substantial number of telephones in the vicinity of any facility likely to receive threatening telephone calls. For example, if an airline has virtually instantaneous-acting means to identify the location of any public telephone station within the airport or within a radius of a few miles of the airport, the source of any threatening call can be located with a reasonable chance that the caller can be apprehended. On the other hand, a security system of this kind capable of identifying the origin of threatening telephone calls cannot be effective if it interferes with normal use of the telephones or gives any readily detectable indication of its presence. Moreover, if a substantial number of telephones must be monitored, it is essential that any equipment added to the transmitting telephones be simple and economical in construction and inconspicuous in size, such that it can be incorporated in the telephone instrument itself without indication of its presence, or be susceptible of installation at an exchange or other location where it cannot be detected. At the receiving station, where threatening calls are to be identified for location of origin, it may also be necessary to add security equipment to a number of telephones, so that the same economic strictures are applicable.
There are other more mundane situations which need verification of the location of origin of a particular communication. For example, a decentralized commercial organization in which it is essential to accommodate business transactions transmitted by telephone line, may require an essentially instantaneous means for determining whether a particular communication originates from a given location. It is common to use specified codes, often changed on a daily basis, for security purposes in a commercial arrangement of this kind. Substantially greater security can be realized, however, if the system has the capability of verifying the origin of messages from all stations in the system.